In optical time division multiplexing (OTDM) systems, each of N optical pulse streams is modulated with information from a respective data stream. Each of the N optical pulse streams is formed from a plurality of optical pulses. The N optical pulse streams are interleaved in time to produce a single optical signal, which is transmitted into an optical network. The optical signal also includes a stream of framing pulses. Thus, the optical signal comprises N+1 optical pulse streams. The framing pulse stream is a stream of periodic pulses that generally do not carry information from a data source but are used as a time reference at the receiving end of the system to facilitate recovery of information from the N information-carrying optical pulse streams.
To generate the N optical pulse streams and the framing pulse stream, a light source, for example, a mode-locked laser, is used to generate a periodic train of pulses with widths on the order of a few tens of picoseconds (ps) or less. The laser pulse width is much shorter than the bit period T, which is the time period between consecutive pulses from the laser. The laser output is split into N+1 channels. Once the laser output has been split into N+1 channels, each of N optical pulse streams is modulated by a respective data source. The framing pulse stream is generally not modulated.
Various optical properties of the pulses of the N optical pulse streams can be modulated to encode information from a digital data source. When light intensity is modulated between at least two values, the modulation technique is amplitude shift keying (ASK). When the phase of the light is modulated between at least two angular values, the modulation technique is phase shift keying (PSK). The N modulated optical pulse streams are then interleaved with one another and with the framing pulse stream to form the optical signal. The N+1 optical pulse streams are interleaved such that pulses of different pulse streams do not overlap with one another.
In a bit interleaved OTDM stream, the ith optical pulse stream, where i=1, 2, . . . , N is delayed by iτ, where τ=T/(N+1). Generally, the pulse width of each pulse is shorter than τ, which is the bit interval in the optical signal. The framing pulse stream is typically undelayed. The delays offset pulses of the N optical pulse streams and framing pulse stream from one another by a time τ. Thus, when interleaved, the pulses of the N delayed data streams and the framing pulse stream do not overlap with one another.
At the receiving end of an OTDM system, the data carried by the N data streams is recovered. Data recovery in OTDM systems includes both selection, which is the separation of a particular channel from the other pulse streams in the optical signal and the demodulation of information from the separated channel. In certain cases, the pulses of the framing pulse stream can be used to assist demodulating a pulse stream. For example, in bit interleaved OTDM systems, sequential pulses of the framing pulse stream are spaced apart by a time period T that is equal to the time period between sequential pulses in each of the N information carrying channels. To demodulate a particular pulse stream, the framing pulses are first offset in time so that each framing pulse is temporally aligned with a corresponding pulse of the channel to be demodulated. Subsequently, a logical AND operation is performed between the pulses of the information carrying channel and the framing pulse stream to select a particular pulse stream to be modulated.
A number of methods have been proposed to perform the logical AND operation. A first method is a nonlinear optical loop mirror (NOLM). A NOLM includes a 3-dB directional coupler and a fiber loop connecting both outputs of the coupler. Another coupler is used for insertion of control pulses. The input signal splits into clockwise and counterclockwise streams after passing the coupler. These streams gain different phase shifts while propagating around the loop due to the non-linearity of silica fiber itself. Depending on the phase shift each stream acquires propagating around the loop, the two streams will experience constructive or destructive interference at the coupler. In NOLM systems it is necessary to either use a long fiber of about a few kilometers or to use pulses having a high peak power in order to get pulse switching because silica fiber possesses only weak non-linearity.
The second configuration is called a terahertz optical asymmetric demultiplexer (TOAD), as disclosed in U.S. Pat. No. 5,493,433 to Prucnal et al. The TOAD includes a separate non-linear element (NLE) in the loop to change the phase shift acquired by each stream. The non-linear properties of the NLE are controlled by control pulses. Therefore, the fiber loop includes another coupler for insertion of control pulses. The separate non-linear element can be, for example, a semiconductor optical amplifier (SOA), which is driven into saturation by the control pulse. The control signal and the first signal must pass the SOA after the second signal passed it. In this case the first signal experiences a phase shift due to the amplifier saturation. Because the resulting interference will not be completely destructive, the device will produce an output signal. In TOAD systems, pulse synchronization and SOA optimization are technically complicated.
Another approach to performing the logical AND operation is by use of a soliton trapping AND gate, such as that disclosed in U.S. Pat. No. 4,932,739 to Islam. This method utilizes the property of solitons with orthogonal polarization states to propagate in a birefringent fiber with the same group velocity. While propagating along the fiber these two solitons are subjected to the wavelength shifts in opposite directions, and the narrowband output selector can register the signal at the shifted wavelength. However, the technique is complicated by the long fiber lengths that must be used to achieve the soliton trapping effect.
Known optical AND gates do not themselves recover information from optical pulse streams. Rather, receivers using such AND gates require an additional element, such as a detector that operates on a direct detection principal, to recover information from the output of the AND gate.
The rate and efficiency of optical communication using TDM systems would greatly benefit by finding a faster and more sensitive approach to demodulating and demultiplexing optical signals encoded as time division multiplexed optical signals.
Coherent optical detection exploits the coherence properties of the transmitted optical signal. Generally, the optical signal is mixed with a signal from a local oscillator, which is typically a laser at the receiving end of the system. Because of the mixing, coherent detection can be modeled as a type of optical interferometry. The signal intensity resulting from the mixed signals is detected by a photoreceiver, such as a photodiode.
An advantage of coherent detection over direct detection is the higher sensitivity at the receiving end in coherent detection. As shown by R. Ramaswami, K. N. Sivarajan “Optical Networks: A Practical Perspective”, 1998, the coherent homodyne receiver sensitivity for bit error rates (BER) of 10−12 is 49 photons per 1 bit, while direct detection pinFET receiver without preamplifier has a sensitivity on the order of a few thousands photons per 1 bit, and preamplified receivers have the sensitivity of a few hundred photons per 1 bit to obtain a BER of 10−12.
A variety of optical schemes have been proposed for coherent optical receivers. One configuration, the single branch receiver, includes a 3-dB coupler (or beam splitter in a free space version), a local oscillator and a photo detector. Improved efficiency is achieved by a configuration using two identical photodiodes connected to two outputs of the coupler as disclosed in U.S. Pat. No. 4,596,052 to Wright et al. The resulting output signal is the difference of the signals from the two photodiodes, and it is equal to that of single branch photoreceiver. The power, however, of the local oscillator is more efficiently used. More advantageous receivers have been developed based upon polarization diversity (U.S. Pat. No. 4,718,120 to Tzeng, U.S. Pat. No. 5,060,312 to Delavaux) and phase diversity (U.S. Pat. No. 5,115,332 to Naito et al., U.S. Pat. No. 5,146,359 to Okoshi et al., and U.S. Pat. No. 5,081,712 to Meissner) ones which are insensitive to fiber-induced fluctuations of the signal polarization state and phase.